It is well known that filters are employed to remove contaminants from liquids or gases so as to purify the same. In the environment of pneumatically controlled instruments, these impurities or contaminants are removed to prevent their failure or ineffective operation. Generally, pneumatically controlled instruments function the same as those of an electric or hydraulic controller. The distinguishing feature of a pneumatic controller is that air pressure is used as the controlling medium instead of electricity or hydraulic pressure. In general, pneumatic controllers are available with the various types of controls--two position, proportional, proportional plus integral, proportional plus derivative, and proportional plus integral plus derivative actions--that are available in electric controllers. These systems are useful in sensing a change in position. As such, the measured variable, when changing, creates differences in force (pressure) rather than actual small movements of an element in space (as in a position balance system).
It is also known that some aircraft instruments are supplied with pneumatic air pressure so that the instruments may be controlled gyroscopically. In particular, a gyro-horizon indicator has a gyro with a plurality of vanes which receives system air pressure at its outer circumference causing the gyro to spin similar to a paddle wheel. The spinning gyro (8 to 10,000 rpm) remains in a fixed position relative to the earth so as to display true vertical and horizontal information regardless of the attitude of the aircraft. Therefore, a horizon bar attached to the gyro always remains parallel with the earth's horizon so as to provide the pilot with a level reference. A directional gyro indicator operates in a similar manner. The directional gyro has a stationary ring attached to the spinning gyro and is marked with various compass degrees. Once established in flight, the gyro is stabilized such that the aircraft literally moves around the stationary fixed gyro. This allows the pilot to determine the direction of the aircraft by reading the compass degrees off the stationary ring.
For the aforementioned pneumatically controlled aircraft instruments to function properly, supply of an unfettered source of pressurized air is required. As such, it is well known to provide a pneumatic air filter between a pressure regulator driven by a pump and the instruments to be controlled. These known pneumatic air filters are only checked on two occasions. The first occasion is when one of the indicators or instruments supplied by the pressure regulator malfunctions or is behaving improperly. The other occasion is when a prescribed maintenance program requires that the pneumatic air filter be changed. The changing of the pneumatic air filter is usually done without regard as to whether the air filter has lost its effectiveness.
It is well known to provide pneumatic air filters in metal housings or other similar rigid structures. Pneumatic air filters contained within metal housings have several positive features. A metal housing provides a sturdy structure that is not easily damaged, can withstand excessive heat, and is effective in preventing contaminates from entering the pneumatic air supply system.
However, pneumatic air filters contained within metal housings have several drawbacks. Primarily, known pneumatic air filters contained within metal housings do not provide a clear indication of when the filter needs to be replaced with a new filter. Nor, do the known air filters provide a visual indication of the amount of contamination between the filter and a failed air pump following an air pump failure.
It is clear that there is a need in the art for an aircraft pneumatic air filter that provides a visible indication of when the filter media is no longer capable of functioning properly. There is also a need in the art for an aircraft pneumatic air filter that is easily maintainable and can withstand the harsh environment associated with the internal control systems of aircraft. There is a further need for an aircraft pneumatic air filter which provides a total cost savings when compared with the known art of such pneumatic air filters.